Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see
Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to
www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Prerequisites for VLANs over IP Unnumbered Subinterfaces

Configure DHCP and ensure that it is operational.

Restrictions for VLANs over IP Unnumbered Subinterfaces

Only Ethernet VLAN subinterfaces, in addition
to serial interfaces, can be configured as IP unnumbered interfaces.

Information About VLANs over IP Unnumbered Subinterfaces

Support for VLANs over IP Unnumbered Subinterfaces

The
VLANs over IP Unnumbered Subinterfaces feature
enables Ethernet VLANs to be configured on IP
unnumbered subinterfaces. The IP unnumbered interface configuration enables IP processing on an interface without
assigning an IP address to the interface. The IP
unnumbered interface borrows an IP address from
another
interface that is already configured on the
device to conserve network and address space.

Figure 1 shows the implementation of the VLANs over IP Unnumbered Subinterfaces feature in a sample
network topology. In this topology, the aggregation services routers dynamically establish
IP routes when the DHCP server assigns IP
addresses to hosts.

The VLANs over IP Unnumbered Subinterfaces feature supports the following functions:

Allocating peer IP address through DHCP.

Configuring IP unnumbered interface support for a range of VLAN subinterfaces.

Configuring service selection gateway support for VLANs over IP unnumbered subinterfaces.

Supporting DHCP relay agent information feature (Option 82).

DHCP Option 82

DHCP provides a framework for passing
configuration information to hosts on a TCP/IP network.
Configuration parameters and other control
information are carried in tagged data items (also called options) that are
stored
in the options field of the DHCP message. Option 82 is
organized
as a single DHCP option that contains
information known by the relay agent.

The DHCP Relay Agent Information feature communicates information to the
DHCP server using a suboption of the DHCP relay agent
information option called agent remote ID. The
information sent in the agent remote ID includes an IP
address identifying the relay agent and
information about the interface and the connection over
which
the DHCP request was received. The DHCP server uses this information to assign IP addresses to interfaces and to form security policies.

Figure 2 shows the
agent remote ID suboption format that is used with the VLANs over
IP Unnumbered
Subinterfaces feature.

Figure 2. Format of the Agent Remote ID Suboption

Field

Description

Type

Format type (1 byte). Value 2 specifies the format for use with this feature.

Length

Length of the agent remote ID suboption (1 byte). The type field and the remaining bytes of the length field are not included.

Executes commands on multiple subinterfaces simultaneously. The device prompt changes to configuration interface range mode after the commands are executed.

Separate the interface range with a hyphen and space as shown in the example.

Step 4

encapsulation dot1qvlan-id[native]

Example:

Device(config-if-range)# encapsulation dot1q 10

Applies a unique VLAN ID to each subinterface within the range.

The VLAN ID specified by the vlan-id argument is applied to the first subinterface in the range. Each subsequent interface is assigned a VLAN ID, which is the specified vlan-id including the subinterface number and excluding the first subinterface number (VLAN ID + subinterface number - first subinterface number).

Step 5

ip unnumberedtype number

Example:

Device(config-if-range)# ip unnumbered ethernet 3/0

Enables IP processing on an interface without assigning an explicit IP address to the interface.

The type and number arguments specify an interface with a predefined IP address on the device. Do not specify an unnumbered interface, if one already exists.

RFCs

RFCs

Title

RFC 1812

Requirements for IP Version 4
Routers, June 1995

Technical Assistance

Description

Link

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Feature Information for VLANs over IP Unnumbered Subinterfaces

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to
www.cisco.com/​go/​cfn. An account on Cisco.com is not required.